Method for making a lithium mixed metal compound having an olivine structure

ABSTRACT

A method for preparing a Li x M y PO 4  compound having an olivine structure includes: preparing a solution containing transition metal M ions, Li +  ions and PO 4   3−  ions; drying the solution to form particles of a starting material; and forming the particles of the starting material into particles of the Li x M y PO 4  compound with an olivine structure, in which 0.8≦x≦1.2 and 0.8≦y≦1.2, and coating the particles of the Li x M y PO 4  compound with a carbon layer thereon.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/222,569 (hereinafter referred to as the '569 application),filed on Sep. 9, 2005 and abandoned as of the filing date of thisapplication. The '569 application claims priority of Taiwaneseapplication no. 094115023, filed on May 10, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for making a lithium mixed metalcompound, more particularly to a method for making a lithium mixed metalcompound having an olivine structure and coated with a carbon layerthereon.

2. Description of the Related Art

Lithium-containing transitional metal compounds, such as layered cobaltcompounds, layered nickel compounds and spinelle manganese compounds,have been developed for use in positive electrode active materials.However, the cobalt compounds, such as lithium cobalt oxide (LiCoO₂),are hardly applied to highly capacitive battery cells due toinsufficient resources and poisonous property. The nickel compounds,such as lithium nickel oxide (LiNiO₂), are difficult to synthesize andare unstable. While the manganese compounds, such as lithium manganeseoxide (LiMnO₂), are expected to be suitable for the highly capacitivebattery cells because they are relatively economical and safe, they havelow capacity, and are unstable and poor in cycle performance. Inaddition, when the cobalt compounds, nickel compounds and manganesecompounds are applied to a battery cell, the initial capacity value ofthe cell will diminish during the first cycle operation and will furtherdiminish upon every successive cycle of operation.

Another lithium-containing transitional metal compound, olivine lithiumferrous phosphate (LiFePO₄), has been considered for use in positiveelectrode active materials. The lithium ferrous phosphate has goodelectrochemical properties, good environmental and operational safety,sufficient resources, high specific capacity, cycle performance, andheat stability. Lithium ferrous phosphate has a slight twisted hexagonalclose-packed structure that includes a framework consisting of FeO₆octahedrals, LiO₆ octahedrals, and PO₄ tetrahedrals. In the structure oflithium ferrous phosphate, one FeO₆ octahedral is co-sided with two LiO₆octahedrals and one PO₄ tetrahedral. However, since the structure ofsuch lithium ferrous phosphate lacks continuous co-sided FeO₆ octahedralnetwork, no free electrons can be formed to conduct electricity. Inaddition, since the PO₄ tetrahedrals restrict lattice volume change,insertion and escape of the lithium ions into and from the lattice oflithium ferrous phosphate are adversely affected, thereby significantlydecreasing the diffusion rate of lithium ions. The conductivity and iondiffusion rate of lithium ferrous phosphate are decreased, accordingly.

Meanwhile, the smaller the particle size of the lithium ferrousphosphate, the shorter will be the diffusion path of the lithium ions,and the easier will be the insertion and escape of the lithium ions intoand from the lattice of lithium ferrous phosphate, which is advantageousto enhance the ion diffusion rate. Besides, addition of conductivematerials into the lithium ferrous phosphate is helpful in improving theconductivity of the lithium ferrous phosphate particles. Therefore, ithas been proposed heretofore to improve the conductivity of the lithiumferrous phosphate through mixing or synthesizing techniques.

Currently, methods for synthesizing olivine lithium ferrous phosphateinclude high temperature-solid state reaction, carbothermal reduction,and hydrothermal reaction. For example, U.S. Pat. No. 5,910,382discloses a method for making olivine compound LiFePO₄ powders bypreparing intimate mixtures of stoichiometric proportions of Li₂CO₃ orLiOH.H₂O, Fe{CH₂COOH}₂ and NH₄H₂PO₄.H₂O, and heating the mixtures in anon-oxidizing atmosphere at an elevated temperature ranging from 650° C.to 800° C. However, the particle size of the resultant LiFePO₄ powdersis relatively large, has an uneven distribution, and is not suitable forcharge/discharge under a high electrical current. In addition, theferrous source, i.e., Fe{CH₂COOH}₂, is expensive, which results in anincrease in the manufacturing costs, accordingly.

U.S. Pat. Nos. 6,528,033, 6,716,372, and 6,730,281 disclose methods formaking lithium-containing materials by combining an organic material anda mixture containing a lithium compound, a ferric compound and aphosphate compound so that the mixture is mixed with excess quantitiesof carbon coming from the organic material and so that ferric ions inthe mixture are reduced to ferrous ions. The mixture is subsequentlyheated in a non-oxidizing atmosphere so as to prepare LiFePO₄ throughcarbothermal reduction. However, the methods provided by these prior artpatents involve addition of a great amount of organic materials to themixture, and excess quantities of carbon in LiFePO₄ tend to reduceferrous ions to iron metal and result in loss of specific capacity.

All the aforesaid methods for making LiFePO₄ involve solid-statereaction and require long reaction time and a high temperaturetreatment. The LiFePO₄ powders thus formed have a relatively largeparticle size, a poor ionic conductivity, and a relatively highdeteriorating rate in electrochemical properties. In addition, theLiFePO₄ powders thus formed are required to be ball-milled due to theirlarge particle size, and the quality of the LiFePO₄ powders willdeteriorate due to impurity pollution.

In addition, the method for making LiFePO₄ through hydrothermal reactionmay use soluble ferrous compound, lithium compound, and phosphoric acidas starting materials, so as to control the particle size of LiFePO₄.However, hydrothermal reaction is relatively difficult to carry outsince it requires to be conducted at a high temperature and a highpressure.

Therefore, there is still a need in the art to provide an economical andsimple method for making a lithium mixed metal compound having arelatively small particle size and good conductivity.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a methodfor making a lithium mixed metal compound that can alleviate theaforesaid drawbacks of the prior art.

According to this invention, a method for preparing a Li_(x)M_(y)PO₄compound having an olivine structure, includes: preparing a solutioncontaining transition metal M ions, Li⁺ ions and PO₄ ³⁻ ions; drying thesolution to form particles of a starting material; and forming theparticles of the starting material into particles of the Li_(x)M_(y)PO₄compound with an olivine structure, in which 0.8≦x≦1.2 and 0.8≦y≦1.2,and coating the particles of the Li_(x)M_(y)PO₄ compound with a carbonlayer thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent in the following detailed description of the preferredembodiments of this invention, with reference to the accompanyingdrawings, in which:

FIG. 1 shows the results of an X-ray diffraction pattern of the LiFePO₄powders prepared according to Example 1 of the present invention;

FIG. 2 shows the results of an X-ray diffraction pattern of the LiFePO₄powders prepared according to Example 2 of the present invention;

FIG. 3 shows the results of an X-ray diffraction pattern of the LiFePO₄powders prepared according to Example 6 of the present invention;

FIG. 4 shows a SEM photograph to illustrate surface morphology of theLiFePO₄ powders prepared according to Example 6 of the presentinvention;

FIG. 5 shows a specific capacity/cycle number plot of a battery cellwith cathode material made from the LiFePO₄ powders prepared accordingto Example 6 of the present invention;

FIG. 6 shows a voltage/capacity plot of a battery cell with cathodematerial made from the LiFePO₄ powders prepared according to Example 6of the present invention; and

FIG. 7 is a schematic view to illustrate how reaction of making alithium mixed metal compound coated with a carbon layer thereon isconducted in a reaction chamber in the first preferred embodiment ofthis invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of a method for preparing a Li_(x)M_(y)PO₄compound having an olivine structure according to this inventionincludes: preparing a solution containing transition metal M ions, Li⁺ions and PO₄ ³⁻ ions; drying the solution to form particles of astarting material; and forming the particles of the starting materialinto particles of the Li_(x)M_(y)PO₄ compound with an olivine structure,in which 0.8≦x≦1.2 and 0.8≦y≦1.2, and coating the particles of theLi_(x)M_(y)PO₄ compound with a carbon layer thereon.

Non-limiting examples of the transition metal M of the transition metalM ions include at least one selected from the group consisting of Fe,Ti, V, Cr, Mn, Co, Ni, and combinations thereof.

Preferably, the formation of the Li_(x)M_(y)PO₄ compound and the carbonlayer on the particles of the Li_(x)M_(y)PO₄ compound are conducted byheating the particles of the starting material in the presence ofsuspended carbon particles.

Referring to FIG. 7, formation of the Li_(x)M_(y)PO₄ compound andcoating of the Li_(x)M_(y)PO₄ compound with the carbon layer thereon areconducted in a reaction chamber 10. The atmosphere in the reactionchamber 10 is preferably a non-oxidizing atmosphere that consists of anon-oxidizing carrier gas.

In another preferred embodiment, the suspended carbon particles areformed by heating a carbonaceous material in the reaction chamber 10 toform carbon particles that are subsequently suspended in the reactionchamber 10 by the non-oxidizing carrier gas introduced into the reactionchamber 10 and passing over the heated carbonaceous material.

Preferably, the non-oxidizing carrier gas is inert to the startingmaterial. Non-limiting examples of the non-oxidizing carrier gas includeat least one selected from the group consisting of nitrogen, argon,carbon monoxide, carbon dioxide, and mixtures thereof. More preferably,the non-oxidizing carrier gas is nitrogen.

Non-limiting examples of the carbonaceous material include at least oneselected from the group consisting of charcoal, graphite, carbonpowders, coal, organic compounds, and mixtures thereof. Preferably, thecarbonaceous material is charcoal.

In yet another preferred embodiment, the heating operation of thecarbonaceous material in the reaction chamber 10 is conducted at atemperature higher than 300° C. Preferably, the carbonaceous material isheated at a temperature ranging from 300° C. to 1100° C. Morepreferably, the carbonaceous material is heated at 700° C.

In addition, the transition metal M ions may be prepared by dissolving apre-mixture in water. Non-limiting examples of the pre-mixture include acompound of the transition metal M, powders of the transition metal Mand an acid, and combinations thereof.

In one preferred embodiment, the pre-mixture is the compound of thetransition metal M. More preferably, the compound of the transitionmetal M is a ferric or ferrous compound selected from the groupconsisting of ferric nitrate (Fe(NO₃)₃), ferric chloride (FeCl₃), andferrous chloride (FeCl₂).

In another preferred embodiment, the pre-mixture includes powders of thetransition metal M and an acid. Preferably, the powders of thetransition metal M are iron powders.

The aforesaid acid may be chosen from one of an inorganic acid and anorganic acid. Non-limiting examples of the inorganic acid include nitricacid (HNO₃), sulfuric acid (H₂SO₄), hydrochloric acid (HCl), perchloricacid (HClO₄), hypochloric acid (HClO₃), hydrofluoric acid (HF),hydrobromic acid (HBrO₃), phosphoric acid (H₃PO₄), and mixtures thereof.In case that the pre-mixture includes the iron powders and one of nitricacid or hydrochloric acid, the transition metal M ions thus formed areferric ions (Fe³⁺). In case that the pre-mixture includes the ironpowders and phosphoric acid, the transition metal M ions thus formed areferrous ions (Fe²⁺).

Non-limiting examples of the organic acid may be selected from the groupconsisting of formic acid (HCOOH), acetic acid (CH₃COOH), propionic acid(C₂H₅COOH), citric acid (HOOCCH₂C(OH)(COOH)CH₂COOH.H₂O), tartaric acid((CH(OH)COOH)₂), lactic acid (CH₃CHOHCOOH), ascorbic acid, and mixturesthereof.

In one preferred embodiment, the Li⁺ ions are prepared from a lithiumcompound. Non-limiting examples of the lithium compound include lithiumhydroxide (LiOH), lithium fluoride (LiF), lithium chloride (LiCl),lithium oxide (Li₂O), lithium nitrate (LiNO₃), lithium acetate(CH₃COOLi), lithium phosphate (Li₃PO₄), lithium hydrogen phosphate(Li₂HPO₄), lithium dihydrogenphosphate (LiH₂PO₄), lithium ammoniumphosphate (Li₂NH₄PO₄), lithium diammonium phosphate (Li(NH₄)₂PO₄), andmixtures thereof. Preferably, the lithium compound is lithium hydroxide(LiOH).

In another preferred embodiment, the PO₄ ³⁻ ions are prepared from aphosphate compound. Non-limiting examples of the phosphate compoundinclude ammonium hydrogen phosphate ((NH₄)₂HPO₄), ammonium dihydrogenphosphate ((NH₄)H₂PO₄), ammonium phosphate ((NH₄)₃PO₄), phosphoruspentoxide (P₂O₅), phosphoric acid (H₃PO₄), lithium phosphate (Li₃PO₄),lithium hydrogen phosphate (Li₂HPO₄), lithium dihydrogen phosphate(LiH₂PO₄), lithium ammonium phosphate (Li₂NH₄PO₄), lithium diammoniumphosphate (Li(NH₄)₂PO₄), and mixtures thereof. Preferably, the phosphatecompound is phosphoric acid (H₃PO₄).

In yet another preferred embodiment, a saccharide is added into thesolution prior to drying of the solution. Non-limiting examples of thesaccharide include sucrose, glycan, and polysaccharides. Morepreferably, the saccharide is sucrose.

The drying of the solution may be conducted using any method known toone skilled in the art. Non-limiting examples of the drying methodinclude oven-drying, spray-drying and the like.

In addition, preferably, formation of the particles of the startingmaterial into the particles of the Li_(x)M_(y)PO₄ compound and coatingthe particles of the Li_(x)M_(y)PO₄ compound with the carbon layerthereon are conducted at a temperature ranging from 400° C. to 1000° C.for 1 to 30 hours. More preferably, formation of the particles of thestarting material into the particles of the Li_(x)M_(y)PO₄ compound andcoating the particles of the Li_(x)M_(y)PO₄ compound with the carbonlayer thereon are conducted at a temperature ranging from 450° C. to850° C. for 4 to 20 hours. Most preferably, formation of the particlesof the starting material into the particles of the Li_(x)M_(y)PO₄compound and coating the particles of the Li_(x)M_(y)PO₄ compound withthe carbon layer thereon are conducted at 700° C. for 12 hours.

EXAMPLES

Reactants and Equipments:

-   1. Ferric nitrate (FeNO₃): commercially obtained from C-Solution    Inc., Taiwan;-   2. Ferric chloride (FeCl): commercially obtained from C-Solution    Inc., Taiwan;-   3. Iron powders: commercially obtained from Hoganas Ltd., Taiwan,    mode no. NC-100.24;-   4. Nitrogen gas (N₂): commercially obtained from C-Solution Inc.,    Taiwan;-   5. Nitric acid (HNO₃): commercially obtained from C-Solution Inc.,    Taiwan;-   6. Hydrochloric acid (HCl): commercially obtained from C-Solution    Inc., Taiwan;-   7. Phosphoric acid (H₃PO₃): commercially obtained from C-Solution    Inc., Taiwan;-   8. Lithium hydroxide (LiOH): commercially obtained from Chung-Yuan    chemicals, Taiwan;-   9. Sucrose: commercially obtained from Taiwan Sugar Corporation,    Taiwan;-   10. Carbon black: commercially obtained from Pacific Energytech Co.,    Ltd., Taiwan;-   11. Polyvinylidene difluoride (PVDF): commercially obtained from    Pacific Energytech Co., Ltd., Taiwan; and-   12. Tubular furnace: commercially obtained from Ultra Fine    Technologies, Inc., Taiwan.

Example 1

0.2 mole of FeCl₂ was added to 200 ml of deionized water. After theFeCl₂ was completely dissolved in the deionized water, 0.2 mole ofphosphoric acid and 100 ml of 2N LiOH solution was then added, so as toform a solution having a stoichiometric ratio 1:1:1 of Fe²⁺:Li⁺:PO₄ ³⁺.The solution was dried into a powdery starting material, and was thenplaced in an aluminum oxide crucible. The crucible together withcharcoal was placed in a tubular furnace which was heated at 700° C. for12 hours in the presence of a nitrogen carrier gas charging into thefurnace. Carbon particles formed from the charcoal were suspended in thenitrogen carrier gas. LiFePO₄ particles coated with a carbon layer wereobtained.

The LiFePO₄ particles coated with the carbon layer were subsequentlyanalyzed by CuKα X-ray diffraction analyzer (manufactured by SGS TaiwanLtd., Taiwan) and the results are shown in FIG. 1. The X-ray patternshown in FIG. 1 demonstrates that the LiFePO₄ particles coated with thecarbon layer have an olivine crystal structure.

Example 2

In this example, the LiFePO₄ particles coated with a carbon layer wereprepared in a manner similar to that of Example 1, except that 0.2 moleof FeCl₂ was replaced with 0.2 mole of FeNO₃.

The LiFePO₄ particles coated with the carbon layer were subsequentlyanalyzed by CuKα X-ray diffraction analyzer, and the results are shownin FIG. 2. The X-ray pattern shown in FIG. 2 demonstrates that theLiFePO₄ particles have an olivine crystal structure.

Example 3

In this example, the LiFePO₄ particles coated with a carbon layer wereprepared in a manner similar to that of Example 1, except that 0.2 moleof FeCl₂ was replaced with a mixture of 0.2 mole of iron powders and 50ml of concentrated HNO₃.

Example 4

In this example, the LiFePO₄ particles coated with a carbon layer wereprepared in a manner similar to that of Example 3, except that 50 ml ofconcentrated HNO₃ was replaced with 100 ml of concentrated HCl.

Example 5

In this example, the LiFePO₄ particles coated with a carbon layer wereprepared in a manner similar to that of Example 3, except that 50 ml ofconcentrated HNO₃ was replaced with 0.2 mole of H₃PO₄.

Example 6

In this example, the LiFePO₄ particles coated with a carbon layer wereprepared in a manner similar to that of Example 5, except that 3.2 g ofsucrose was added to the reactant mixture before the reactant mixturewas dried and heated.

The LiFePO₄ particles coated with the carbon layer were subsequentlyanalyzed by CuKα X-ray diffraction analyzer and observed by scanningelectron microscope (SEM), and the results are shown in FIGS. 3 and 4,respectively. The X-ray pattern shown in FIG. 3 and the photograph shownin FIG. 4 demonstrate that the LiFePO₄ particles have an olivine crystalstructure and a particle size of about 100 nm.

Example 7

A mixture containing the LiFePO₄ particles obtained from Example 6,carbon black, and polyvinylidene difluoride (PVDF) in a ratio of 83:10:7was prepared and mixed thoroughly. The mixture was subsequently coatedon a piece of aluminum foil and was dried to form a cathode. The cathodewas applied to a battery cell, and the battery cell was subjected to acharge/discharge test in a charge/discharge tester. The battery cell wascharged and discharged at an approximate C/5 (5 hour) rate at a voltageranging from 2.5 V and 4.5 V. The results of specific capacity variationare shown in FIG. 5. The results of voltage variation at the charge anddischarge plateau in the 15^(th) cycle at room temperature are shown inFIG. 6. According to the results shown in FIG. 5, the initial specificcapacity of the battery cell at room temperature is about 148 mAh/g,while after thirty cycles of charge/discharge operations, the specificcapacity of the battery cell at room temperature reaches about 151mAh/g. These results demonstrate that the battery cell has a good cyclestability. According to the results shown in FIG. 6, thecharge/discharge performance and stability are improved.

In view of the foregoing, high temperature and high pressure operationsutilized in the conventional methods are not required in the method ofthis invention. Besides, compared with the LiFePO₄ powder productobtained from the conventional methods, the LiFePO₄ particles coatedwith a carbon layer obtained according to the method of the presentinvention have a smaller particle size and more uniform particle sizedistribution, and the ball-milling treatment required in theconventional method can be omitted. Therefore, the method of thisinvention is more economical than the conventional methods in terms ofproduction cost.

Additionally, by virtue of coating the LiFePO₄ particles with the carbonlayer, the LiFePO₄ particles obtained according to the method of thepresent invention have enhanced electrical conductivity and capacity.Specifically, through coating with the carbon layer, the electricalconductivity of the LiFePO₄ particles will increase from around 10⁻⁸ MHOto around 10⁻⁴ MHO and the capacity of the LiFePO₄ particles willincrease from around 10 mAh to around 140 mAh.

While the present invention has been described in connection with whatare considered the most practical and preferred embodiments, it isunderstood that this invention is not limited to the disclosedembodiments but is intended to cover various arrangements includedwithin the spirit and scope of the broadest interpretation andequivalent arrangements.

1. A method for preparing a Li_(x)M_(y)PO₄ compound having an olivinestructure, the method comprising: preparing a solution containingtransition metal M ions, Li⁺ ions and PO₄ ³⁻ ions; drying the solutionto form particles of a starting material; and forming the particles ofthe starting material into particles of the Li_(x)M_(y)PO₄ compound withan olivine structure, in which 0.8≦x≦1.2 and 0.8≦y≦1.2, and coating theparticles of the Li_(x)M_(y)PO₄ compound with a carbon layer thereon. 2.The method of claim 1, wherein the transitional metal M of thetransition metal M ions is at least one selected from the groupconsisting of Fe, Ti, V, Cr, Mn, Co, Ni, and combinations thereof. 3.The method of claim 1, wherein the formation of the Li_(x)M_(y)PO₄compound and the carbon layer on the particles of the Li_(x)M_(y)PO₄compound is conducted by heating the particles of the starting materialin the presence of suspended carbon particles.
 4. The method of claim 3,wherein the suspended carbon particles are formed by heating acarbonaceous material in a reaction chamber to form carbon particleswhich are subsequently suspended in the reaction chamber by anon-oxidizing carrier gas introduced into the reaction chamber andpassing over the heated carbonaceous material.
 5. The method of claim 4,wherein the non-oxidizing carrier gas is at least one selected from thegroup consisting of nitrogen, argon, carbon monoxide, carbon dioxide,and mixtures thereof.
 6. The method of claim 4, wherein the carbonaceousmaterial is at least one selected from the group consisting of charcoal,graphite, carbon powders, coal, organic compounds, and mixtures thereof.7. The method of claim 4, wherein the heating operation of thecarbonaceous material is conducted at a temperature ranging from 300° C.to 1100° C.
 8. The method of claim 1, wherein the transition metal Mions are prepared by dissolving a pre-mixture in water, the pre-mixtureincluding a compound of the transition metal M.
 9. The method of claim8, wherein the compound is a ferric compound selected from the groupconsisting of ferric nitrate and ferric chloride.
 10. The method ofclaim 8, wherein the compound is a ferrous compound selected from thegroup consisting of ferrous chloride.
 11. The method of claim 1, whereinthe transition metal M ions are prepared by dissolving a pre-mixture inwater, the pre-mixture including powders of the transition metal M andan acid.
 12. The method of claim 11, wherein the powders are ironpowders.
 13. The method of claim 11, wherein the acid is an inorganicacid selected from the group consisting of nitric acid, sulfuric acid,hydrochloric acid, perchloric acid, hypochloric acid, hydrofluoric acid,hydrobromic acid, phosphoric acid, and mixtures thereof.
 14. The methodof claim 11, wherein the acid is an organic acid selected from the groupconsisting of formic acid, acetic acid, propionic acid, citric acid,tartaric acid, lactic acid, ascorbic acid, and mixtures thereof.
 15. Themethod of claim 1, wherein the Li⁺ ions are prepared from a lithiumcompound selected from the group consisting of lithium hydroxide,lithium fluoride, lithium chloride, lithium oxide, lithium nitrate,lithium acetate, lithium phosphate, lithium hydrogen phosphate, lithiumdihydrogen phosphate, lithium ammonium phosphate, lithium diammoniumphosphate, and mixtures thereof.
 16. The method of claim 1, wherein thePO₄ ³⁻ ions are prepared from a phosphate compound selected from thegroup consisting of ammonium hydrogen phosphate, ammonium dihydrogenphosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid,lithium phosphate, lithium hydrogen phosphate, lithium dihydrogenphosphate, lithium ammonium phosphate, lithium diammonium phosphate, andmixtures thereof.
 17. The method of claim 1, further comprising adding asaccharide into the solution prior to drying of the solution.
 18. Themethod of claim 17, wherein the saccharide is selected from the groupconsisting of sucrose, glycan, and polysaccharides.
 19. The method ofclaim 1, wherein formation of the particles of the starting materialinto the particles of the Li_(x)M_(y)PO₄ compound and coating theparticles of the Li_(x)M_(y)PO₄ compound with the carbon layer thereonare conducted at a temperature ranging from 400° C. to 1000° C. for 1 to30 hours.
 20. The method of claim 19, wherein formation of the particlesof the starting material into the particles of the Li_(x)M_(y)PO₄compound and coating the particles of the Li_(x)M_(y)PO₄ compound withthe carbon layer thereon are conducted at a temperature ranging from450° C. to 850° C. for 4 to 20 hours.